The present invention relates to uninterruptible power supplies and in particular to an apparatus and a method for testing the integrity of isolation means used in an uninterruptible power supply to isolate an auxiliary power source from a main power supply during a disruption of the main power supply.
An uninterruptible power supply (herein referred to as a “UPS”) can be used to guarantee the power supply to an electrical load for a limited period in the event of a disruption to the dedicated mains electricity supply. A UPS is generally provided with an input for the mains electricity supply, an output for connection to the electrical load, a rechargeable energy storage unit, such as a battery, interconnected with the output, and a switch located between the input and the interconnected energy storage unit/output network, and a sensor detecting any disruption to the electricity supply at the input. In normal operation (e.g. non-disruption of mains power), the switch is closed and permits energy to flow from the input to the energy storage unit and the output. In emergency operation (e.g. mains power failure), the sensor detects the disruption to the electricity supply at the input and activates the switch to isolate the energy storage unit/output network from the input. Accordingly the power supply to the electrical load is maintained by the energy storage unit.
Such a UPS is described in Japanese Patent Unexamined Publication No. 04-147076 which also includes a backup-battery check circuit to detect a failure of the rechargeable battery (energy storage unit).
It is vitally important during emergency operation that the energy storage unit is always isolated from the disrupted mains electricity network since otherwise it would rapidly discharge the vast majority of its stored energy back into the mains network and thus would be incapable of maintaining the power supply to the electrical load for any appreciable length of time.
In most instances, the isolation capability of the switch is only ever tested during actual real-life situations when the UPS is called upon to switch from normal operation to emergency operation. Obviously, if the switch fails to isolate the energy storage unit from the input in these circumstances there could be catastrophic consequences when the power supply to the electrical load unexpectedly plummets and eventually fails.
A UPS is described in European EP-A-0309124 which during emergency operation detects whether an electronic isolation switch has failed, and if so, operates a mechanical isolation backup switch to isolate the entire UPS circuit from the commercial AC input. However, this purely reactive procedure is only brought into effect during emergency operation when the commercial AC power supply has already been disrupted. As such it requires the additional mechanical isolation backup switch to ensure that power is prevented from flowing from the battery into the disrupted commercial AC power supply.